It has to accomplish a few things:
1. List enough attack powers to make monsters (most monsters, anyway), all animals, and a decent number of "energy" or "mystical" attacks.
2. It has to have rules for playing very large and very small characters.
3. It has to have a wide variety of each category of ability--there will not just be 'Armor' but different kinds of Armor-plates and other rules around similar defenses to give the character and style.
One of the first sections is "body types." I wanted to make sure that non-human animal-style characters would have some options that would give them basic templates to work from. For example: Quadruped (fast) gives the character superior stability and higher ground speed. They have a lower shoulder-height than a human of the same BLD and, unless they do something special, have no hands (but four legs!).
The Centaur Issue
I have a list of all or at least most of these abilities with "stat blocks" (although they are not "Complete and up-to-date"). One of the early ones is Centaur Body Type which is a humanoid torso (arms and head) and a lower body of something else (snake? horse? spider? etc.).
The purpose of the body-type is two-fold: (1) to let the character "play that thing" but secondly (2) to separate the JAGS-sian generalities around "Big things." JAGS is pretty dedicated to making big things "big." If you play an intelligent Elephant in JAGS you are going to have hundreds of Damage Points. If you play a "horse-man" you're going to have "a lot of DP."
And you're going to be big too. And bigger things hit harder in JAGS.
But there's a problem: in our formulation a Centaur with a sword who weighs 1500 lbs doesn't hit as hard as a 1500lb Giant with a sword--so says I. I mean, they could: and if you think they do, fine.
But they don't have to. The human torso's arms will not be as strong as the horse's legs. Presumably the arms can't lift a fraction of 1500 lbs as the legs can (and jump it). Furthermore, while a centaur might train to get their weight into a strike it seems unlikely that the kinematics of a horse will match up as well as those of a full on body twisting sword swing.
I mean--maybe not: I'm not an expert--but it seems to me that a centaur with a sword would strike like a very strong man--but not as well as a 1500lb sword using giant (although if you do assume weapons better created for the centaur they would get the advantage of their mass).
CENTAURS IN JAGS
If you are buying the Centaur Body Type you have two options. The first is that you treat it just like a human. This is the case if you think that a Centaur with a sword hits as hard as the presumably optimized giant. In that case you just buy the stability / ground move. Maybe a Trample attack and hooves. And you're more or less good to go.
But if you take the other option then what? The other option is this:
You get a Centaur Body Type which limits the damage you do with punches and weapons to roughly that of the humanoid part (plus a bit) and you use your less optimized attacks (trample, mule kick) with your horse-portion. This means that the Centaur pays less for its size than a "Giant humanoid" of the same size since the horse-attacks (Trample, Mule-Kick) are less efficient in terms of REA and other things than punches or sword blows.
If you do that, then it is cheaper to be large like a centaur. This is good--there are all kinds of things out there that are not as efficient in terms of weapon usage as a human. Take, for example, a shark: it has a charge-in-and-bite attack. This is a powerful attack but the damage comes primarily from its jaw strength and not from the "ram attack" it does. A great white shark can weigh 4200 lbs. However if it swims up and bites you, while often fatal it is not always fatal the same way that JAGS would treat a human of that size swinging a sword at you.
What To Do?
What we are doing is this: providing a list of "body types" (Quadruped, Pisciform (fish), Avian, Insectoid, Serpent) and giving them some "basic attacks" that are usually less efficient than punches and human kicks. This makes their size and strength cheaper. If they do take something that optimizes their weight then they need to pay the same price as everyone else.
How'd I Come Up With Those Forms?
It turns out that there are (I checked) six phylum of animals--and under that a whole host of "orders of animals." I went through those and tried to get categories that made sense for the game. Especially since there will be some weird combinations (air sharks?) and the game categorization is not the scientific one (reptiles are a different phylum than mammals but crocodiles, cats, dogs, and horses are all four-legged creatures with different bio-weapons, skin-coverings, diets, and movement abilities).